Solid oxide fuel cells (“SOFC's”) and associated fuel processors are known. SOFC's are solid-state devices which use an oxygen ion conducting ceramic electrolyte to produce electrical current by transferring oxygen ions from an oxidizing gas stream at the cathode of the fuel cell to a reducing gas stream at the anode of the fuel cell. This type of fuel cell is seen as especially promising in the area of distributed stationary power generation. SOFC's require an operating temperature range which is the highest of any fuel cell technology, giving it several advantages over other types of fuel cells for these types of applications. The rate at which a fuel cell's electrochemical reactions proceed increases with increasing temperature, resulting in lower activation voltage losses for the SOFC. The SOFC's high operating temperature can preclude the need for precious metal catalysts, resulting in substantial material cost reductions. The elevated exit temperature of the flow streams allow for high overall system efficiencies in combined heat and power applications, which are well suited to distributed stationary power generation.
The traditional method of constructing solid oxide fuel cells has been as a large bundle of individual tubular fuel cells. Systems of several hundred kilowatts of power have been successfully constructed using this methodology. However, there are several known disadvantages to the tubular design which severely limit the practicality of its use in the area of 25 kW-100 kW distributed stationary power generation. For example, producing the tubes can require expensive fabrication methods, resulting in achievable costs per kW which are not competitive with currently available alternatives. As another example, the electrical interconnects between tubes can suffer from large ohmic losses, resulting in low volumetric power densities. These disadvantages to the tubular designs have led to the development of planar SOFC designs. The planar designs have been demonstrated to be capable of high volumetric power densities, and their capability of being mass produced using inexpensive fabrication techniques is promising.
As is known in the art, a single planar solid oxide fuel cell (SOFC) consists of a solid electrolyte which has high oxygen ion conductivity, such as yttria stabilized zirconia (YSZ); a cathode material such as strontium-doped lanthanum manganite on one side of the electrolyte, which is in contact with an oxidizing flow stream such as air; an anode material such as a cermet of nickel and YSZ on the opposing side of the electrolyte, which is in contact with a fuel flow stream containing hydrogen, carbon monoxide, a gaseous hydrocarbon, or a combination thereof such as a reformed hydrocarbon fuel; and an electrically conductive interconnect material on the other sides of the anode and cathode to provide the electrical connection between adjacent cells, and to provide flow paths for the reactant flow streams to contact the anode and cathode. Such cells can be produced by well-established production methodologies such as screen-printing and ceramic tape casting.
However, there are still challenges to implementing the planar SOFC for stationary power generation in the range of 25 kW-100 kW. The practical size of such cells is currently limited to a maximum footprint of approximately 10×10 cm by issues such as the thermal stresses within the plane of the cell during operation and the difficulties involved in fabricating very thin components. Since the achievable power density of the fuel cell is in the range of 180-260 mW/cm2, a large number of cells must be assembled into one or more fuel cell stacks in order to achieve the required power levels for a stationary power generation application. Implementing large numbers of such cells presents several difficulties. A planar SOFC design requires high-temperature gas-tight seals around the edges of the cells, which typically requires large compressive loads on the stack. Anode and cathode flowstreams must be evenly distributed among the many cells. The heat generated by the fuel cell reaction must be able to be removed from the stack in order to prevent overheating. These issues and others have made it difficult for planar SOFC manufacturers to progress to fuel cell systems larger than about 5 kWe.
Thus, while the known systems may be suitable for their intended purpose, there is always room for improvement.